-LRB- CNN -RRB- -- Two tech companies are offering a free tool to defeat sophisticated malicious software that criminals have used to extract millions of dollars in ransom payments from small businesses and other computer owners .

The software from FireEye and Fox IT can help victims decrypt files that were locked by the malware Cryptolocker .

The scheme typically worked this way : Victims would click a seemingly legitimate email , which automatically downloaded the malware on their computers .

Upon infection , victims would lose access to their files and criminals would demand payment in order to regain access .

The Justice Department announced in June it had taken down the Cryptolocker malware .

The malware remains defunct , but there are many victims whose files were locked by criminals and remain held hostage .

Since emerging in 2013 , Cryptolocker has been used to attack about 200,000 computers , half of which were in the United States . In its first two months , criminals extorted an estimated $ 27 million from victims .

David Hickton , the U.S. attorney in Pittsburgh whose office led the investigation , said in June that Cryptolocker victims included the police department in Swansea , Massachusetts .

It paid a ransom to cyber criminals to restore access to its files after its systems were infected .

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Companies offer free tool to defeat malicious software used by criminals

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They have extracted millions of dollars in ransom from small businesses

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So-called Cryptolocker has been used to attack about 200,000 computers